Dressed to the Nines: Caps

Spalding’s Base Ball Guide for 1888

"The baseball cap is the quintessential American hat. It is seen not just on the ball field, but in the stands, on the streets and even at the workplace. Today, as in the past, the baseball cap serves many functions. It shades a player’s eyes from the sun during the day and from artificial lights at night. It helps players and fans distinguish members of one team from another. A cap’s distinctive coloring and graphics help promote the team and make for an enticing retail product. And how could a club mount a late-inning comeback without displaying an inside-out 'rally cap?' Old Hat. On April 24, 1849, the New York Knickerbockers, baseball’s seminal organization, adopted the first official uniform. The first baseball caps were chip (or straw) hats. A few years later the club switched to a cap made of merino (a soft, fine wool) that featured the two main characteristics of the modern-day baseball cap: a crown and a bill (or visor)."
Dressed to the Nines: Caps

"... Caps, or other types of headgear with eye-shades, have been a part of baseball uniforms from the beginning. From the 1840s to the 1870s, baseball players wore various types of hats, or even no cap at all, since there was no official rule regarding headgear. Examples included full-brimmed straw hats such as boating caps, jockey caps, cycling caps, and flat-topped caps. The Brooklyn Excelsiors was the first team to wear what would later become the modern baseball cap, with its distinctive rounded top and peak, in the 1860s. By the early years of the twentieth century, this style of cap had become common, but some teams occasionally revived the flat-topped cap, such as the New York Giants in 1916 and the Pittsburgh Pirates as recently as during the 1979 World Series."
Wikipedia